Ask Ian: Progressive Twist (Gain Twist) in Small Arms?

From Jamie on Utreon:
"I know naval gatling guns like the 20mm CIWS uses gain twist rifling, but are or were there any commercial or military small arms that used gain twist rifling?"

Yes, there have been some significant uses of progressive (gain) twist rifling on military and civilian small arms. These include:

Colt percussion revolvers (1848, 1849, 1851, 1860, 1861 models)
Custom barrels for Sharps, Stevens, and other rifles by Harry Pope
Carcano Model 1891 rifles and carbines
S&W .460 XVR revolvers

There are certainly others, but those are some of the most significant.

Contact:
Forgotten Weapons
6281 N. Oracle 36270
Tucson, AZ 85740

[tehsnekychicken]

This format for QnAs is great - I liked watching the full-length, and the quality of the answer was obviously similar, but this is so much easier to go back and reference. Easier to remember that "progressive rifling" was talked about in "progressive rifling", rather than #87 or something

[emergingloki]

Must admit I'm really enjoying this format. Still loving the guns, but this related information running in parallel is fascinating.
With Ian, every day really is a school day.

[starkindustries]

Stuff like this is super interesting to me, I’d like to see more “nerdy mechanical engineering” stuff on small arms throughout history

[PetrHosek]

One thing to remember is that the bullet is accelerating as it travels through the barrel, so even with a constant rate rifling the rotation of the bullet increases somewhat gradually.

[AllAboutSurvival]

I didnt know there was such a thing called gain twist rifling. I still have a lot to learn about guns and thanks to Ian, I have subscribed to this channel.. I’ve been learning a lot!

[lawrencehudson]

One of the items you mentioned was the effect of the twist on the body of the bullet and you were including or at least from what I heard including the 20 MM in this. Now I will admit that my knowledge is dated with my experience ending 1976 but up to then the 20 MM rounds all had copper alloy rotating bands and the steel body was essentially a bore riding design. The gain twist on these is more from the centrifical fuze of some of the designs which depend on the spin of the projectile to line up the elements of their explosive chain and make the rounds safer to handle. Newer rounds like the 30 MM for the GAU 8 have a nylon composition rotating band. All in all, a fine video as always just I felt I had to point out something I felt needed more clarification.

[johnhufnagel]

The intuitive part of my brain also says, progressive twist should help cut down on erosion of the initial rifling, thereby extending the life of the barrel. Perhaps not as important in hand guns, but I could potentially see it increasing longevity for expensive barrels for rifles of various calibers.

[prjndigo]

to point: all bullets traveling down a rifle "gain twist" in that they move forward first and it takes a short amount of travel in the barrel to gain that spin from the rifling. Variations in chamber pressure can have effects on how quickly they gain.

[heyhayhay]

That shirt is perfection. A normal person won't notice anything beyond a tropical shirt and never realize it's camo.

[slendsunny]

In long range competition shooting community, gain twist is some what getting popular, especially in 264cal and 308cal. Many 6.5 CM shooters uses 24-28 inch barrel with twist gain from 1:9 (chamber)to 1:7 (muzzle), claimed more stable grouping and slower throat erosion.

[mikecanaday]

During development of the M39 and M61 gain twist rifle also showed a slight increases in muzzle velocity over uniform twist rifling. The GAU-12 25mm, and the two 30mms, GAU-8 and M230, also use gain twist rifling. Some people claim gain twist is used for fusing reason, but this is not the case. The Army's M50 series 20mm ammunition for the gain twist M39 and M61 uses the same fuses as the Navy's Mk 100 20mm ammunition for the Mk 12 cannon, which has uniform twist rifling. Also, large artillery, 105mm, 155mm, etc., use uniform twist rifling, and these have much more intricate and complex fuses.

[skenzyme]

Imagine a mechanical "pre spin" system that got the whole cartridge turning before firing.

I bet Textron considered it for their NGSW submission before deciding such a design would be too simplistic.

[Kaboomf]

On Colt revolvers with gain twist:

I own and sometimes shoot an original '51 Navy revolver with this feature, and have looked into it somewhat. To my understanding, the goal was to achieve decent accuracy with both round ball and much heavier conical bullets. Even today, if you shop for repro muzzleloaders you find that ones optimised for heavy conical projectiles tend not to shoot well with round ball and vice versa. The issue is longer conicals need a higher rifling twist for stability, whereas round ball have such a small rifling engagement area that steep twist rifling can just skip over the bullet instead. There's no cylindrical bullet shank to engage the rifling, only a narrow band of lead touches the bore. So if you try to shoot round ball with a full service charge of powder in a barrel meant for conicals, the rifling may just strip away parts of the bullet instead of spinning it up in a predictable manner. Colt's solution sort of works, there's less force on that narrow engagement area so they shoot well enough (but not super accurate, in my experience) with both bullet types.

[maxcactus]

IIRC, Harry Pope (precision barrel maker) was the dean of gain twist rifling. He was doing it in the early 20th century.

[Jake-dpif]

The mostly 3d printed improvised PDW, the FGC-9, uses a homemade barrel with progressive rifling. Most likely to better suit the improvised cast lead ammunition in countries where jacketed bullets are not readily available

[davidgillon]

Progressive depth rifling was used in the UK 's late WWII 3.7" MkVI AA gun where Colonel Probert reduced the last few inches of the barrel to smooth bore, eliminating the barrel wear that affected the MkIV (they were both using a 4.5" case to push a 3 .7" shell) and improving the ballistic performance.

[georgeadams]

I really enjoy this format, I feel you can go deeper into each question. Thank you.

[vz__skorpion]

Another modern gun that uses it is the diy FGC-9 MKii! I don't believe there was any real purpose for using it other than showing what could be done using the diy electro-chemical machining techniques thay were developed, but that seems good enough of a reason to me.

[charlesphillips]

The only variable depth rifling I have heard of is in large guns where the rifling starts deep and finishes smooth bore. This is to reduce drag by smoothing the driving band into the body of the shell.

[semibreve]

The new format is awesome, but I absolutely love the ability to chuck on a hour long Q&A video and just listen along and relax. Please don't completely give up on the longform!